In electric and electronic equipment in recent years, specifically, a radio-frequency printed board or inverter-related equipment, for example, a fast switching device, an inverter motor or an electric equipment coil for a transformer or the like, demands have been made for further improving various kinds of performance, for example, heat resistance, mechanical properties, chemical properties, electric properties and reliability in comparison with a conventional product. For such electric and electronic equipment, an insulated wire, which is an enameled wire, has been mainly used as a magnet wire. For a polymer insulating material used for the insulated wire, a low relative dielectric constant and high heat resistance together with high insulation properties have been demanded.
In particular, in the insulated wire such as an enameled wire to be used as the magnet wire for electric and electronic equipment for space use, electric and electronic equipment for aircraft use, electric and electronic equipment for nuclear power use, electric and electronic equipment for energy use, electric and electronic equipment for vehicle use, demands have been made for a high partial discharge inception voltage and also excellent insulation performance under high temperature as required properties for the insulation properties, and excellent thermal aging-resistant properties under high temperature as one of required properties for heat resistance.
By the way, inverters have been installed in many types of electric and electronic equipment, as an efficient variable-speed control unit. However, inverters are switched at a frequency of several kHz to tens of kHz, to cause a surge voltage at every pulse thereof. Inverter surge is a phenomenon that reflection occurs at a breakpoint of impedance, for example, at a starting end, a termination end, or the like of a connected wire in the propagation system, followed by applying a voltage up to twice as high as the inverter output voltage. In particular, an output pulse occurred due to a high-speed switching device, such as an IGBT, is high in steep voltage rise. Accordingly, even if a connection cable is short, the surge voltage is high, and voltage decay due to the connection cable is also low. As a result, a voltage almost twice as high as the inverter output voltage occurs.
As described above, since a voltage almost twice as high as the inverter output voltage is applied in inverter-related equipment, demands have been made for minimizing an inverter surge deterioration of the enameled wire (also referred to as insulated wire), which is one of the materials constituting the coils of those electric and electronic equipment.
In general, partial discharge deterioration means a phenomenon in which the following deteriorations of the electrical insulating material occur in a complicated manner: molecular chain breakage deterioration caused by collision with charged particles that have been generated by partial discharge (discharge at a portion in which fine void defect exists); sputtering deterioration; thermal fusion or thermal decomposition deterioration caused by local temperature rise; and chemical deterioration caused by ozone generated due to discharge, and the like. The electrical insulating materials which actually have been deteriorated by partial discharge show reduction in the thickness.
It has been believed that inverter surge deterioration of an insulated wire also proceeds by the same mechanism as in the case of general partial discharge deterioration. Namely, inverter surge deterioration of an enameled wire is a phenomenon in which partial discharge occurs in the insulated wire due to the surge voltage with a high peak value, which occurs at the inverter, and the coating of the insulated wire causes partial discharge deterioration as a result of the partial discharge; in other words, the inverter surge deterioration of an enameled wire is high-frequency partial discharge deterioration.
Insulated wires that are able to withstand several hundred volts of surge voltage have been demanded for the recent electric and electronic equipment. That is, there is a need for insulated wires that have a partial discharge inception voltage of 500 V or more. Herein, the partial discharge inception voltage is a value that is measured by a commercially available apparatus called partial discharge tester. Measurement temperature, frequency of the alternating current voltage to be used, measurement sensitivity, and the like are values that may vary as necessary, but the above-mentioned value is an effective value of the voltage at which partial discharge occurs, which is measured at 25° C., 50 Hz, and 10 pC.
When the partial discharge inception voltage is measured, a method is used in which the most severe condition possible in the case where the insulated wire is used as a magnet wire is envisaged, and a specimen shape is formed which can be observed in between two closely contacting insulated wires. For example, in the case of an insulated wire having a circular cross-section, two insulated wires are brought into linear contact by spirally twisting the wires together, and a voltage is applied between the two insulated wires. Alternatively, in the case of an insulated wire having a rectangular cross-section, use is made of a method of bringing two insulated wires into planar contact through the planes, which are the long sides of the insulated wires, and applying a voltage between the two insulated wires.
In order to obtain an insulated wire that does not cause partial discharge, which means having a high partial discharge inception voltage, so as to prevent the deterioration of the insulated layer (also referred to as “enamel layer”) of the insulated wire caused by the partial discharge, it is thought to utilize a method of using a resin having low specific permittivity in the enamel layer or increasing the thickness of the enamel layer.
An attempt has been actually made on decreasing a relative dielectric constant of an enamel resin (Patent Literatures 1 and 2). However, the relative dielectric constant of the resin or the insulating layer as described in Patent Literatures 1 and 2 is only 3 to 4. In order to adjust the partial discharge inception voltage of the insulated wire using the resin or the insulating layer to 1 kV or more (effective value), experience shows that a thickness of the insulating layer is required to be adjusted to 100 μm or more, and room for a further improvement is left in view of the partial discharge inception voltage.
In addition, to thicken the insulating film, the number of times for passing through a baking furnace increases in a production process thereof, whereby making a film composed of copper oxide on a copper conductor surface thicker, this in turn, causing lowering of adhesion between the conductor and the backed enamel layer. For example, in the case of obtaining an enamel layer with thickness 100 μm or more, the number of passing through the baking furnace exceeds 20 times. It has been known that if this number of passages exceeds times, the adhesive force between the conductor and the enamel layer is conspicuously lowered.
It is also thought to utilize a method of increasing the thickness that can be formed by a single baking step, in order not to increase the number of passing through the baking furnace. However, this method has a drawback that the solvent of the varnish dose not completely vaporize and remains in the enamel layer as voids.
In order to increase a thickness of an insulation coating, an attempt has also been made on arranging a coating resin outside an enameled wire using a thermoplastic resin having a low relative dielectric constant (Patent Literatures 3 and 4). However, the relative dielectric constant of a synthetic resin for forming an insulating layer as used in Patent Literature 3 is at a level same as the level described above. Even if the insulating layer of an insulated wire is formed using the synthetic resin described in Patent Literature 3, the performance is far from fully satisfactory in view of the partial discharge inception voltage, the insulation performance and thermal aging resistance under high temperature.
To solve the problems, an attempt has been made on applying a thermosetting resin having cells to an insulation coating (Patent Literatures 5 to 8). However, even if the thermosetting resins described above are used for an insulating film, room for a further improvement is left in view of any one of the partial discharge inception voltage, the dielectric breakdown properties and the heat resistance.